1. Field of the Invention
The present invention relates to a process and apparatus wherein poly substituted benzenes are transalkylated to form additional mono substituted product. More particularly the invention relates to a process for the alkylation of benzene where polyalkylated benzenes are by-products which are transalkylated in a separate apparatus and process step.
2.Related Information
Ethyl benzene and cumene have traditionally been produced by the reaction of benzene and the respective olefin, i.e., ethylene and propylene in the presence of an acidic catalyst. In some known processes the catalyst is highly corrosive and has a relatively short life, e.g., A1C13, H3P04 on clay, BF3 on alumina, and others require periodic regeneration, e.g., molecular sieves. The exothermicity of the reaction tends to produce poly substituted benzene. In the case of ethyl benzene production the poly substituted benzenes are diethyl benzenes. In the case of cumene production the poly substituted benzenes comprise poly isopropyl benzenes (PIPB) which include di-isopropyl benzenes (DIPB""s). In addition heavier compounds may also be produced.
The poly substituted benzenes may be transalkylated with additional benzene utilizing the same catalyst as the alkylation catalyst to produce more of the mono substituted benzene. It has been observed that there is an optimum per pass conversion of poly substituted benzene which minimizes impurity formation and results in improved yield and better purity product. The poly substituted benzene conversion is significantly affected by various combinations of temperature and residence time. The various reaction chemistry involved is very complex and is presently not fully understood. Some of the reactions benefit from high temperature and low residence time, and others are affected adversely by too long a residence time. In the past, however, overexposure to excess catalyst could not be avoided, since in most commercial designs, catalyst reserve (excess) was provided to account for catalyst aging so that the desired run length could be attained.
The present invention comprises providing a transalkylation reactor with multiple beds and feed points so that-unneeded catalyst can be by-passed until such time as the aging of the catalyst required the additional catalyst. To prevent contamination of the unused beds by the reactants, the unused beds may be flooded with clean benzene (which is a required reactant in the transalkylation reaction) and a slight flow of the benzene through the unused beds into the active beds maintained.